1. Field of the Invention
The present invention relates to a position measuring apparatus for obtaining position information by processing two sinusoidal signals with the phase difference of 90.degree. obtained by a detecting device in accordance with the change of a position and further to an encoder by the use of the position measuring apparatus and a surveying apparatus by the use of the encoder.
2. Related Background Art
A conventional position measuring apparatus is shown in FIG. 1. Two sinusoidal signals having the phase difference of 90.degree. are output from a detecting device 1. The sinusoidal signals are then amplified by an amplifier 2 and converted by a wave-shaping device 3 into rectangular signals which are supplied to a counting circuit 4. The counting circuit 4 counts the number of rectangular signals to obtain rough position information and supplies it to a processing circuit 5. On the other hand, the two sinusoidal signals with the phase difference of 90.degree. amplified by the amplifier 2 are sent to an A/D converter 6 to be digitized and thereafter supplied to the processing circuit 5. The processing circuit 5 inputs the digitized two sinusoidal signals with the phase difference of 90.degree. in a tan.sup.-1 table stored in a ROM to obtain minute displacement. Further, the processing circuit 5 obtains position information by adding this minute displacement and the above rough position information. Then, the position information is displayed on a displaying portion 7 as well as output to an external output portion 8. The detailed description of the conventional position measuring apparatus in FIG. 1 is disclosed in Japanese Patent Laid-Open Application No. 56-96213.
However, in the above-mentioned conventional position measuring apparatus, there is a problem that the accuracy of measurement is deteriorated by the fluctuation of accuracy of input sinusoidal signals.
If first and second sinusoidal signals A and B with the phase difference of 90.degree. obtained from the detecting device 1 have respective amplitudes a and b and the phase error is .alpha., both the signals A and B are expressed as: EQU A=a.times.cos.theta.+C1, (1) EQU B=b.times.sin(.theta.+.alpha.)+C2 (2)
However, when in order to obtain the position represented by the angle .theta., the first and second sinusoidal signals A and B are input to the A/D converter 6 and the angle .theta. is converted into the position by means of the tan.sup.-1 table, a position. error is produced owing to the difference between the amplitudes a, b, the phase difference .alpha. and the difference between the offsets C1, C2. If a, b, .alpha., C1 and C2 were constant, the processing and conversion could be performed by handling these as correction constants. But not so, this operation is impossible.